This invention generally relates to color picture tubes and, more particularly, to an apparatus for maintaining tension in a shadow mask of a cathode ray tube.
A conventional color picture tube includes an electron gun for generating and directing three electron beams to the screen of the tube. The screen is located on the inner surface of the faceplate of the tube and is made up of an array of elements of three different color-emitting phosphors. A color selection electrode, also referred to as a shadow mask, is interposed between the gun and the screen to permit each electron beam to strike only the phosphor elements associated with that beam. A shadow mask is a thin sheet of metal, such as steel, that is usually contoured to somewhat parallel the inner surface of the tube faceplate.
One type of color picture tube shadow mask is a tension mask mounted within a faceplate panel thereof. In order to maintain the tension on the mask, the mask must be attached to a relatively massive support frame. Although such tubes have found wide consumer acceptance, there is still a need for further improvement, to reduce the weight and cost of the mask-frame assemblies in such tubes.
It has been suggested that a lighter frame could be used in a tension mask tube if the required tension on a mask is reduced. One way to reduce the required mask tension is to make the mask from a material having a low coefficient of thermal expansion. However, a mask from such material requires a support frame of a material having a similar coefficient of thermal expansion, to prevent any mismatch of expansions during thermal processing that is required for tube manufacturing, and during tube operation. Because the metal materials that have low coefficients of thermal expansion are relatively expensive, it is costly to make both the mask and frame out of identical or similar low expansion materials. Therefore, it is desirable to use the combination of a low expansion tension mask with a higher expansion support frame, and to provide a solution to the problem that exists when there is substantial mismatch in coefficients of thermal expansion between a tension mask and its support frame.
The present invention provides an apparatus for maintaining tension in a shadow mask. The invention compensates for the thermal expansion mismatch between a low expansion material such as INVAR(copyright) which is an alloy chiefly composed of Ni and Fe, and a high expansion material such as steel used in the tension mask and the mask frame, respectively. The need to maintain tension in a tension mask is essential to the correct operation of the cathode ray tube in a color television.
In the present invention, the mask frame has a generally rectangular plan form with two long sides and two short sides. A corner support structure incorporating a standoff is attached to each end of each short side. Atop the standoffs is welded the long side member comprised of a triangular shaped cross-section support structures referred to herein as triboxes. The first side (leg) of the triangular triboxes are attached to the standoff and the hypotenuse faces inwardly of the mask frame. The second side (leg) of the triangular triboxes is affixed to a side of a low expansion material, such as INVAR(copyright), blade. The blade is vertically oriented and the top of the blade supports an INVAR(copyright) mask. Attached between the short side steel mask frame and the steel triboxes is a brace formed of a low expansion material such as INVAR(copyright). The thermal expansion mismatch and geometric separation between the low coefficient of thermal expansion brace and the steel short side of the mask frame causes the steel tribox to rotate about its stand-off when heated. The brace is sized such that the inward rotation of the tribox compensates for the expansion of the frame, thereby preserving the tension in the mask that is supported by the blade as the frame is heated. This system is specifically designed to maintain tension in the shadow mask during thermal cycling of the mask frame assembly such that uniform creep of the mask occurs and mask wrinkles caused by differential mask creep are eliminated.